Catheter systems utilizing light-based, optical fiber strain sensors to determine touching forces on a distal extremity of an end effector have found favor in recent years for the exploration and treatment of various organs or vessels with catheter-based diagnostic and treatment systems. Such light-based systems can be configured so that they are do not affect and are not affected by electromagnetic radiation environments.
One such light-based catheter system is described in U.S. Pat. No. 6,470,205 to Bosselman which describes a robotic system for performing surgery comprising a series of rigid links coupled by articulated joints. A plurality of Bragg gratings are disposed at the articulated joints so that the bend angle of each joint may be determined optically, for example, by measuring the change in the wavelength of light reflected by the Bragg gratings using an interferometer.
An article by J. Peirs et al., entitled “Design of an Optical Force Sensor for Force Feedback during Minimally Invasive Robotic Surgery,” published by Katholieke Universiteit Leuven, Belgium, describes a tri-axial force sensor for use generating force feedback systems in a robotic surgery system. The apparatus includes a plurality of optical fibers that direct light onto a mirrored surface disposed adjacent to a distal tip of the device. The intensity of the light reflected from the mirrored surface is measured and may be correlated to the force required to impose a predetermined amount of flexure to the distal tip. The article describes a flexible and compact structure that may be used to produce variations in light intensity responsive to contact forces that deform the structure.
International Publication No. WO 2007/015139 to Leo, et al. (Leo), discloses a device and method for resolving a force vector (magnitude and direction) applied to the distal end of a catheter. Leo discloses the use of optical fiber strain elements in a catheter without increasing the profile of the catheter and is substantially immune to electromagnetic interference.
Generally, optical fiber strain sensors are sensitive to changes in temperature. For example, fiber Bragg grating (FBG) sensors include a fiber optic with uniformly spaced gratings formed or etched thereon. Light propagating through the fiber optic is reflected back by the gratings across a narrow wavelength bandpass. The bandpass of the reflected light is related to the spacing of the gratings in accordance with diffraction theory. The spacing is affected not only by the elastic strain experienced by the FBG, but also by thermal contraction and expansion of the FBG relative to a reference state. Temperature changes may also alter the refractive index of the FBG, further affecting wavelength bandpass that is reflected by the grating.
Another example of an optical fiber strain sensor that is generally sensitive to temperature is a Fabry-Perot strain sensor. Fabry-Perot strain sensors include a gap between the end of a transmitting fiber and a reflector. The transmitting fiber is often set up to be partially reflective. Light that enters the gap is inter-reflected between the reflector and the partially reflective transmitting fiber. The signal returned by a Fabry-Perot strain sensor is modulated in accordance with interference theory caused by the inter-reflections. The modulation is related to the dimension of the gap. The gap is affected not only by elastic strain of the structure that defines the gap, but also by thermal contraction and expansion of the structure relative to a reference state.
While optical fiber strain sensors can provide advantages for certain types of catheter procedures, the use of such strain sensors can be negatively impacted in situations involving temperature changes proximate the distal end of the catheter. What is needed is a device and method that adequately compensates for changes in the thermal state of strain sensing catheters utilizing optical fiber strain sensors.